This invention relates to an electrochromic display device, which is a device which operates to produce a display by the electrochemically reversible deposition of coloured species on selected display electrodes.
In British patent specification No. 1,376,399, viologen has been suggested as a suitable electrochromic material, in particular, heptyl viologen dibromide which is colourless in solution but on passage of electric current "plates out" a purple species on the display electrode. However, the present invention is not restricted to this material, practice of the invention being of general application. The type of display to which the invention applies is one in which groups of display electrodes are connected together for the purpose of selecting at which electrodes the display effect is to be achieved. For example, in one common type of display, many dot-shaped electrodes are arranged in rows and columns. A selected display image is generated by directing current to selected display electrodes so that the image is formed by a pattern of coloured dots. For selection and isolation of the display electrodes, each electrode is associated with a three terminal electronic switch such as a transistor. All control electrodes of the switches associated with respective rows of display electrodes are connected to respective row conductors. The display electrodes of each column are connected in parallel through their respective switches and a respective column conductor to drive and selection circuitry for that column. It is this parallel connection of display electrodes in groups in an electrochromic display which gives rise to the problem which practice of the present invention aims at avoiding. The group in the arrangement just described is the display electrodes of the same column but it can be any arrangement in which current paths exist between a plurality of display electrodes. Thus, groups of display electrodes may be connected together so that selected letters or digits may be displayed by selecting some of the electrodes of the connected group. For example, a rectangular array of 7.times.9 dot-shaped display electrodes can form a single character position, all the electrodes being connected through respective transistor switches to a common drive circuitry. The transistor control electrodes can be connected to selection circuitry such that letters or numbers can be displayed by colouring of selected display electrodes in response to energizing selected control electrodes.
As described in British patent specification No. 1,376,399, the coloured species is formed at the display electrode, acting as a cathode, by reduction of uncoloured viologen, the reverse process taking place at a counter electrode, the anode. The formation of coloured species at a display electrode is called herein a write operation, and the removal of such species, by the process of oxidation, an erase operation. A display electrode carrying coloured species will be referred to as written.
When a display electrode is written, sufficient charge is supplied to provide a clearly visible deposit of coloured species. This involves coating the electrode with some tens of molecular layers of the species. One or two layers of the species are not visible. Since the amount of coloured species deposited at an electrode depends on the amount of charge supplied to the electrode, an electrochromic display device is particularly suitable for providing variable intensity displays because the colour shade at an electrode is accurately controllable. A written electrode is at a well-defined potential, given by the Nernst equation EQU E=E.sub.o +k.multidot.1n (p/c)
where E.sub.o and k are constants, c is the bulk concentration of electrochromic material in the electrolyte, and p is the proportion of electrode covered by the deposited species. It follows that E is a maximum when the electrode is fully covered and does not change however much further species is deposited on the electrode. In contrast, an unwritten electrode is at an undefined potential, less than E, determined by the components of the electrolyte.
A potential difference thus exists between the written and unwritten electrodes and, among those electrodes connected in a group through electronic switches, transfer of charge from the written to the unwritten electrodes takes place, against the resistance of the electronic switches, and continues until the unwritten electrodes carry a monolayer of electrochromic material and the potentials of the written and unwritten electrodes are equalized. As charge is removed from the written electrodes, the species deposited on these electrodes is returned to solution. Although the monolayers of electrochromic material on the unwritten electrodes are invisible, the charge transfer, which is called the charge leakage herein, has the undesirable effect of bleaching the display on the written electrodes. The amount of bleaching is not constant since it depends on the ratio of written to unwritten display electrodes in a group. For example, if 1000 equal area display electrodes are connected in a group, if it requires 50 micro-Coulombs/cm.sup.2 to deposit a monolayer of electrochromic material, and if 2.5 milli-Coulombs/cm.sup.2 are used to write at an electrode, and if less than 20 electrodes are written, the charge leakage will dissipate the written charge to result in an invisible monolayer on the written electrodes. However, even if more than 20 electrodes are written, the bleaching effect reduces the contrast between written and unwritten electrodes, especially in those groups of electrodes where only a few are written and, where clear contrast is usually desirable.
The problem of charge leakage can also be regarded from the point of view of the circuit designer. In order to achieve a desired contrast between written and unwritten electrodes, it would be necessary to know how many unwritten electrodes there are in each group in order to determine the charge to be supplied to the electrodes to be written. For a given group of electrodes, this number would vary from image to image. The drive circuitry must then contain current generators which are accurately controllable to deliver varying amounts of current according to the image to be displayed. Such a requirement greatly increases the complexity and cost of the drive circuitry.